A belt drive for accessories generally comprises a drive pulley connected to a crankshaft of the internal-combustion engine of a motor vehicle, at least one second pulley and one third pulley connected respectively to an alternator and to an accessory, for example a hydraulic pump, and a belt for connecting the pulleys together.
In order to start the engine in a fast and silent way, in the engineer it is necessary for the torque supplied by the electric machine to be very high, and this need is particularly felt in motors of the so-called “start and stop” type, which include a starter-alternator, i.e., in some cases a torque of up to 70-90 Nm. Consequently, upstream and downstream of the alternator there is a difference of tensile force on the belt. It is possible to calculate the amount of the difference in tensile force via the ratio between the torque and the radius of the pulley of the alternator. If we assume the aforesaid value of 90 Nm as torque and that the diameter of the pulley of the alternator is 0.054 m, we obtain a value of difference of tensile force of 90 Nm/0.027 N=3300 N, which is a very high value to which the belt is subjected at each engine starting. Furthermore, to be added to said difference of tensile force are the effects of the inertial torques.
In addition, during the starting step, until the combustion has reached the steady-state condition, there occur irregularities of combustion that induce a fluctuating torque on the belt drive.
Said fluctuating torque interacts with the inertia of the accessories driven in rotation by the belt drive, and in particular with that of the alternator, which is the accessory with the highest inertia. The consequent stresses can jeopardize the duration of the belt. This problem is particularly felt in motor vehicles provided with starter-alternator assemblies which are turned off and started again at each stoppage of the vehicle, and in which consequently the belt is required to withstand a very high number of cycles of starting of the vehicle. In particular, according to the requirements of automotive manufacturers, the belts of the starter-alternator assemblies must in fact withstand even up to 700 000 starts without presenting failure.
The resistant inserts are arranged in the belt not exactly in a longitudinal direction, and hence generally there are four resistant inserts in the length of the belt, which do not form a complete circle within the belt, i.e., two inserts per side of the belt no longer form a complete loop that covers the entire length of the belt internally. For example, the situation may arise as illustrated schematically in FIG. 1, where the reference numbers 1 and 2 designate two of the four resistant inserts that do not form a complete loop that covers the entire length of the belt internally.
On the resistant inserts that do not form a complete loop, there is no longer exerted a balancing force in the opposite direction, but rather it is the mix forming the body of the belt that pulls these resistant inserts and undergoes deformation.
On account of the peaks of tensile force, the deformation of the mix and hence of the belt is very high and, even though the adhesion of the resistant insert to the mix surrounding it is high, the aforesaid resistant inserts that do not form a loop that covers the entire length of the belt internally eventually tend to come out of the belt according to the so-called “cord-pop-out phenomenon”, which consequently leads to a decay of the mix in a short time and eventually to failure of the belt.
Finally, on account of the different stiffness, belts with resistant inserts made of different materials undergo an elongation that is markedly different. Consequently, considering only the contribution of the rubber, the generation of heat due hysteresis is very different for resistant inserts made of different materials.
Belts including resistant inserts operate at very high temperatures in steady state running conditions. The high temperature leads to an elongation, which hence results in an increase in creep of the belt.
In addition, known resistant inserts are generally made of polyester, PET, or polyaramide.
Both resistant inserts made of polyester and those made of polyaramide present a high degree of elongation and consequent problems of decay of the ultimate strength over time.